1. Field of the Invention
The present invention relates to a heat-sensitive type flow sensor for detecting a flow rate of a fluid by using heat-sensitive resistors.
2. Description of Related Art
In a conventional heat-sensitive type flow sensor known heretofore, a bridge circuit is employed which is constituted by a plurality of resistance elements inclusive of a first heat-sensitive resistor for detecting an atmospheric or ambient temperature and a second heat-sensitive resistor which is disposed within a passage through which a fluid flows such as, for example, an intake pipe of an internal combustion engine and heated electrically. To this end, a heating current supplied to the second heat-sensitive resistor for electrically heating it is so controlled that the temperature thereof is held higher than the ambient temperature by a predetermined value, wherein the quantity of heat which is deprived of the heat-sensitive resistor by the fluid whose flow rate is to be measured is detected in terms of change of the heating current flowing through the second heat-sensitive resistor. Thus, the flow rate of the fluid such as the intake air can be detected on the basis of the change of the heating current as detected.
For having better understanding of the present invention, description will first be made in some detail of a conventional heat-sensitive type flow sensor. FIG. 5 is a circuit diagram showing a circuit configuration of a hitherto known heat-sensitive type flow sensor such as disclosed, for example, in Japanese Unexamined Patent Application Publication No. 117436/1995 (JP-A-7-117436). Referring to the figure, the conventional heat-sensitive type flow sensor is comprised of a temperature control circuit 10, an operational amplifier circuit 15, a first constant current circuit 16, a second constant current circuit 17, a first constant current control circuit 18, a third constant current circuit 19, a fourth constant current circuit 20 and a second constant current control circuit 37A, details of which will be described below.
At first, description will be directed to the temperature control circuit 10. As can be seen in FIG. 5, the temperature control circuit 10 includes a bridge circuit constituted by resistors R.sub.1 and R.sub.2, a flow rate detecting heat-sensitive resistor R.sub.h and an ambient temperature detecting heat-sensitive resistor R.sub.c, wherein a junction f between the resistor R.sub.1 and the ambient temperature detecting heat-sensitive resistor R.sub.c and a junction b between the resistor R.sub.2 and the flow rate detecting heat-sensitive resistor R.sub.h of the bridge circuit are connected, respectively, to input terminals of a differential amplifier 101 which has an output terminal connected to the base of a transistor 102, the emitter of which is connected to a junction a between the flow rate detecting heat-sensitive resistor R.sub.h and the ambient temperature detecting heat-sensitive resistor R.sub.c while the collector of the transistor 102 is connected to a positive or plus electrode of a DC power source 103 having the other electrode connected to the ground potential.
Next, description will turn to operation of the temperature control circuit 10. When the voltages at the junctions b and f become equal to each other, the bridge circuit assumes an equilibrium or balanced state. In this state, an electric current I.sub.h corresponding to the flow rate of a fluid concerned flows through the flow rate detecting heat-sensitive resistor R.sub.h. The output voltage V.sub.H at the junction b can be given by a product of the current I.sub.h and the resistance value of the resistor R.sub.2. This voltage V.sub.H is made use of as a flow rate signal.
With the view to compensating for dispersions in the flow-rate detection due to dispersions of resistance values of the heat-sensitive resistors R.sub.h and R.sub.c and the resistors R.sub.1 and R.sub.2 as well as temperature coefficients thereof, the detection output value at a predetermined flow rate (ordinarily a relatively low flow rate) is set as a target or desired value by adjusting the resistance value of the resistor R.sub.1 for thereby changing or translating the detection characteristic of the heat-sensitive type flow sensor correspondingly.
Description will now turn to the operational amplifier circuit 15 which is designed to process the flow rate signal outputted from the temperature control circuit 10. The operational amplifier circuit 15 includes an operational amplifier 106 having an inverting input terminal and an output terminal interconnected by way of a feedback resistor R.sub.13 and an input resistor R.sub.11 having one end connected to the junction b of the bridge circuit mentioned above. The other end of the input resistor R.sub.11 is connected to the non-inverting input terminal of the operational amplifier 106.
The first constant current circuit 16 includes a transistor 110 having an emitter coupled to a line of a reference source voltage V.sub.ref by way of a resistor R.sub.25 and a collector connected to the non-inverting input terminal of the operational amplifier 106. On the other hand, the second constant current circuit 17 includes a transistor 111 having an emitter electrode connected to the line of the reference source voltage V.sub.ref by way of a resistor R.sub.26 and a collector connected to the inverting input terminal of the operational amplifier 106. The base electrodes of both the transistors 110 and 111 are connected in cascade and connected in common to an output terminal of an operational amplifier 108 which constitutes a part of the first constant current control circuit 18 which will be described below.
The first constant current control circuit 18 mentioned above is so designed as to control the output current values I.sub.16 and I.sub.17 of the first and second constant current circuits 16 and 17, respectively, on the basis of the preset reference source voltage V.sub.ref. To this end, the first constant current control circuit 18 is constituted by resistors R.sub.20, R.sub.21, R.sub.22, R.sub.23 and R.sub.24 and an operational amplifier 108 connected in such a manner as can be seen in FIG. 5.
Further, the heat-sensitive type flow sensor includes the third constant current circuit 19 of a structure similar to that of the first constant current circuit 16, the fourth constant current circuit 20 implemented in an essentially same structure as that of the second constant current circuit 17 and the second constant current control circuit 37A implemented similarly to the first constant current control circuit 18.
Next, description will turn to operation of the operational amplifier circuit 15. The value or voltage level of the input voltage V.sub.p applied to the non-inverting input terminal of the operational amplifier 106 can be determined by subtracting from the output voltage V.sub.H of the temperature control circuit 10 a voltage drop making appearance across the resistor R.sub.11 due to the current I.sub.16 flowing through the resistor R.sub.11 by way of the first constant current circuit 16. Namely, the input voltage V.sub.p mentioned above can be given by the following expression (1): EQU V.sub.p =V.sub.H -(R.sub.11.times.I.sub.16) (1)
On the other hand, the value or voltage level of the input voltage V.sub.n applied to the inverting input terminal of the operational amplifier 106 can be determined by subtracting from the output voltage V.sub.o of the operational amplifier circuit 15 a voltage drop making appearance across the resistor R.sub.13 due to the current I.sub.17 flowing through the resistor R.sub.13 by way of the second constant current circuit 17. Namely, the input voltage V.sub.p mentioned above can be given by the following expression (2): EQU V.sub.n =V.sub.o -(R.sub.13.times.I.sub.17) (2)
The operational amplifier 106 controls the output voltage V.sub.o of the operational amplifier circuit 15 such that the condition given by V.sub.p =V.sub.n is satisfied. Thus, the output voltage V.sub.o of the heat-sensitive type flow sensor can be given by the following expression (3): EQU V.sub.o =V.sub.H -(R.sub.11.times.I.sub.16)+(R.sub.13.times.I.sub.17) (3)
In conjunction with the above expression (3), it is noted that when the resistance values of the resistors R.sub.11 and R.sub.13 are selected appropriately, e.g. R.sub.11 =R.sub.13, the output voltage V.sub.o of the operational amplifier circuit 15 can be expressed as follows: EQU V.sub.o =V.sub.H -(I.sub.17 +I.sub.16).times.R.sub.11 (4)
Next, operations of the first constant current circuit 16, the second constant current circuit 17 and the first constant current control circuit 18 will be described. Since the input voltage V.sub.20 applied to the non-inverting input terminal of the operational amplifier 108 is derived from the voltage division of the output voltage V.sub.H of the temperature control circuit 10 by the resistors R.sub.20 and R.sub.21, the input voltage V.sub.20 is given by the following expression (5): EQU V.sub.20 ={R.sub.21 /(R.sub.20 +R.sub.21)}.times.V.sub.H (5)
On the other hand, the input voltage V.sub.22 applied to the inverting input terminal of the operational amplifier 108 is derived through voltage division of a potential difference between an inter-terminal voltage V.sub.24 appearing across the resistor R.sub.26 and the preset reference source voltage V.sub.ref by the resistors R.sub.22 and R.sub.23. Accordingly, the input voltage V.sub.22 can be determined in accordance with the following expression (6): EQU V.sub.22 =R.sub.22 /(R.sub.22 +R.sub.23).times.V.sub.24 +R.sub.23 /(R.sub.22 +R.sub.23).times.V.sub.ref (6)
The operational amplifier 108 controls the first and second constant currents I.sub.16 and I.sub.17 flowing through the resistor R.sub.24 by controlling the base currents of the transistors 110 and 111, respectively, and further controls the inter-terminal voltage V.sub.24 of the resistor 24 so that the condition V.sub.20 =V.sub.22 can be satisfied. Accordingly, the following expression (7) holds true. EQU R.sub.21 /(R.sub.20 +R.sub.21).times.V.sub.H =R.sub.22 /(R.sub.22 +R.sub.23).times.V.sub.24 +R.sub.23 /(R.sub.22 +R.sub.23).times.V.sub.ref (7)
From the expression (7), the inter-terminal voltage V.sub.24 of the resistor R.sub.24, i.e., voltage appearing across the resistor R.sub.24, can be determined in accordance with the following expression (8): EQU V.sub.24 =R.sub.21 /(R.sub.20 +R.sub.21).times.(R.sub.22 +R.sub.23)/R.sub.22.times.V.sub.H -R.sub.22 /R.sub.23.times.V.sub.ref (8)
In conjunction with the above expression (8), it is noted that when the resistances of the resistors R.sub.20, R.sub.21, R.sub.22 and R.sub.23 are dimensioned appropriately, e.g. R.sub.20 =R.sub.21 and R.sub.22 =R.sub.23, then the expression (8) can be simplified as follows: EQU V.sub.24 =V.sub.H -V.sub.ref (9)
Since the sum of the first and second constant currents, i.e., I.sub.16 +I.sub.17, is equal to a sum of the current I.sub.24 flowing through the resistor R.sub.24 and the current I.sub.23 flowing through a series circuit of the resistors R.sub.23 and R.sub.22, i.e., I.sub.24 +I.sub.23, the sum of the first and second constant currents, i.e., I.sub.16 +I.sub.17, is given by the following expression (10): EQU I.sub.16 +I.sub.17 =V.sub.24 /R.sub.24 +(V.sub.24 -V.sub.ref)/(R.sub.22 +R.sub.23) (10)
In this conjunction, the following relations apply valid. EQU V.sub.24 =V.sub.H -V.sub.ref and R.sub.22 =R.sub.23 (11)
Accordingly, the sum of the first and second constant currents, i.e., I.sub.16 +I.sub.17, is given by the following expression (12): EQU I.sub.16 +I.sub.17 =(1/R.sub.24 +1/2R.sub.22).times.V.sub.H -(1/R.sub.24 +1/R.sub.22).times.V.sub.ref (12)
With the circuit configuration shown in FIG. 5, only the source current originating in the base currents of the transistors 110 and 111 is of significance to the output of the operational amplifier 108 without being accompanied by any sink current. Further, since the source current mentioned above becomes zero so long as the input voltage V.sub.20 applied to the non-inverting input terminal of the operational amplifier 108 and the input voltage V.sub.22 applied to the inverting input terminal thereof satisfy the condition that V.sub.22.gtoreq.V.sub.20, the first constant current I.sub.16 as well as the second constant current I.sub.17 is zero so long as the condition that V.sub.22.gtoreq.V.sub.20 is satisfied.
Accordingly, the sum of the first and second constant currents, i.e., I.sub.16 +I.sub.17, becomes zero on the conditions that R.sub.20 =R.sub.21 and R.sub.22 =R.sub.23. Thus, the following expression (13) holds true: EQU V.sub.24 +V.sub.ref.gtoreq.V.sub.H (13)
In that case, from the expression (10) and in view of the condition that R.sub.22 =R.sub.23, the voltage V.sub.24 can be given by the following expression (14): EQU V.sub.24 ={R.sub.24 /(2R.sub.22 +R.sub.24)}.times.V.sub.ref (14)
The inter-terminal voltage V.sub.24 can not assume voltage level which is not higher than the value given by the above expression. Accordingly, on the condition given by the expression (15), the following expression (16) holds true. EQU {1+R.sub.24 /(2R.sub.22 +R.sub.24)}.times.V.sub.ref.gtoreq.V.sub.H (15), EQU and EQU I.sub.16 +I.sub.17 =0 (16)
Next, ratios or relation between the first constant current I.sub.16 and the second constant current I.sub.17 will be considered. Both the bases of the transistors 110 and 111 are at a same voltage level because they are connected in common to the output terminal of the operational amplifier 108, as mentioned previously. Accordingly, assuming that the base-emitter voltage V.sub.be of the transistor 110 and that of the transistor 111 are equal to each other, the inter-terminal voltage V.sub.25 appearing across the resistor R.sub.25 is equal to the inter-terminal voltage V.sub.26 which makes appearance across the resistor R.sub.26. Thus, the relations given by the following expressions (17) applies valid. EQU V.sub.25 =I.sub.16.times.R.sub.25 EQU V.sub.26 =I.sub.17.times.R.sub.26 EQU V.sub.25 =V.sub.26 (17)
From the expression (17), the relation given by the following expression (18) applies valid. EQU I.sub.16 =(R.sub.26 /R.sub.25).times.I.sub.17 (18)
From the expressions (12) and (18), difference between the first constant current and the second constant current can be determined in accordance with the following expression (20): ##EQU1##
where G.sub.1 represents R.sub.26 /R.sub.25.
From the above expression (20) and the expression (4) concerning the output voltage V.sub.o of the operational amplifier circuit 15, relation given by the following expression (21) can apply valid. ##EQU2##
As is apparent from the above expression (21), the output voltage V.sub.o of the operational amplifier circuit 15 depends on the output voltage V.sub.H of the temperature control circuit 10 and becomes equal to the output voltage V.sub.H when the condition given by the following expression (22) is satisfied. EQU V.sub.H.ltoreq.{1+R.sub.24 /(2R.sub.22 +R.sub.24)}.times.V.sub.ref (22)
Further, when the condition given by the following expression (23): EQU V.sub.H &gt;{1+R.sub.24 /(2R.sub.22 +R.sub.24)}.times.V.sub.ref (23)
is satisfied, a value which depends on the difference between the output voltage V.sub.H and the preset reference source voltage V.sub.ref, the resistance values of the resistors R.sub.11, R.sub.22 and R.sub.24 and the values of the ratio G.sub.1 between the resistors R.sub.25 and R.sub.26 are added to or subtracted from the output voltage V.sub.H in dependence on the magnitude or value of the above-mentioned ratio G.sub.1 with reference to zero.
As will be appreciated from a foregoing description, in the conventional heat-sensitive type flow sensor, the circuit constants for the constant current control circuits are determined on the presumption that the base-emitter voltage V.sub.be of the transistors 110 and 111 constituting the constant current circuits 16 and 17, respectively, are equal to each other and that the terminal voltages appearing across the resistors connected to the emitters of the transistors, respectively, are equal to each other.
However, in many of the heat-sensitive type flow sensors known heretofore, the transistors 110 and 111 constituting parts of the constant current circuits 16 and 17, respectively, are mounted as discrete components. Consequently, it is practically very difficult or impossible to ensure same operation characteristics for these transistors. In other words, the characteristics of these transistors will unavoidably differ from one to another. As a consequence, the output voltage V.sub.o of the operational amplifier circuit can not necessarily bear correspondence relation to the output voltage V.sub.H derived from the temperature control circuit with a reasonably sufficient fidelity. In order to eliminate these inconveniences, it is required to implement the first and second constant current circuits as well as the third and fourth constant current circuits as the integrated circuits to thereby uniformize the characteristics of these transistors. However, attempt for implementing the constant current circuits in the integrated circuit will encounter another problem that the cost involved in manufacturing the heat-sensitive flowmeter increases unprofitably.